Tag Archives: hops

Report from the 5th Young Scientists Symposium in Chico (Part 2)

It took a little longer than expected to finish the second part of my summaries of the presentations at the 5th International Young Scientists Symposium on Malting, Brewing and Distilling, but here they are. You can find the first part here. I will hopefully have time to post the final part next week. I will also be posting a summary of my own presentation on new lager yeast hybrids.

  • Screening for the brewing ability of non-Saccharomyces yeasts by Maximilian Michel

Maximilian talked about the use of non-conventional yeasts for beer production and he had screened a range of non-Saccharomyces yeasts for brewing potential. Yeast isolates were first identified with genetic fingerprinting and RT-qPCR, and then sent through an initial screening test, which included growth on various carbon sources (glucose, fructose, sucrose, maltose, maltotriose and melibiose), hop resistance (various concentrations of iso-alpha acids), ethanol tolerance (various concentrations of ethanol) and phenolic off-flavour production. Promising strains were then chosen for 2L fermentations. He had focused especially on Torulaspora delbrueckii (but he had also looked at Schizosaccharomyces pombe, Pichia anomala, Hanseniaspora uvarum, Kluyveromyces lactis and Kluyveromyces marxianus), and out of the ten strains he had fermented with at ‘larger’ scale, only one was able to use maltose (and maltotriose). That strain also produced a fruity and berry-like flavour profile. So there are definitely gems to be found in the vast range of wild yeast that are available in nature.

  • Lachancea thermotolerans in primary beer fermentations by Jen House

Jen continued on the topic of using wild yeast in beer fermentations. Her research was on the use of Lachancea thermotolerans, which is an interesting species because of its ability to produce lactic acid. Hence, there is potential to use it in pure culture fermentations for the production of sour beer. Jen had tested three different strains of various origins in wort fermentations, and found that all three were able to use maltose, but not maltotriose. The three strains also produced more lactic acid and glycerol than the S. cerevisiae control. They also seemed to have quite low O2 requirements and were resistant to iso-alpha acids up to at least 60 IBU, which makes them interesting for brewing use. The pH only dropped to around 4.2 in her experimental fermentations, which means that they will only produce a mildly tart beer and may not be suitable for sour beers (as the only microbe). Lachancea yeasts have been isolated from the bark of oak trees, so that may be a good place to start looking in case you are interested in trying to isolate your own!

  • Biodiversity of yeast and lab population isolated from Beninese African Sorghum Beer Starter by Sedjro Emile Tokpohozin

Emile has been looking at the biodiversity of Beninese sorghum beers by isolating yeasts and lactic acid bacteria from starter cultures brought from Benin. These starter cultures aren’t made from pure yeast cultures, rather a small amount of beer from the previous batch is used as a starter culture for the next. Emile had isolated (identification by ITS-PCR and MALDI-TOF-MS) a range of yeasts (e.g. Saccharomyces cerevisiae, Candida krusei, Candida ethanolica and Debaryomyces hansenii) and lactic acid bacteria (Lactobacillus plantarum, Lactobacillus fermentum, Lactobacillus helveticus, Lactobacillus brevis and Lactobacillus paracasei) from a starter culture, and he further screened these for the ability to use various carbon and nitrogen sources, as well as beta-glucosidase ability (in order to break down the cyanogenic compound dhurrin that is found in sorghum). Several possible candidate isolates were identified and these are to be used in some pilot-scale fermentations next. Again shows how much ‘wild’ microbes are out there that are potentially useful in brewing!

  • Invited Speaker: Yeast culture collections by Kyria Boundy-Mills

This talk was a bit different, as Kyria talked about the Phaff yeast culture collection (of which she is the curator of). The Phaff collection is the fourth largest in the world, and contains thousands of yeasts. Many of the deposited yeasts have not been characterized very well, so Kyria talked about the possibility of finding ‘hidden gems’ in the collection. These could have some very interesting properties and phenotypes, relevant not only to the brewing industry, but also e.g. the biofuel industry (oleaginous yeast).

  • Relationships between the speed of fermentation and levels of flavor compounds post-fermentation by Maria Josey

Maria had examined the beer aroma compounds and modelled the fermentation kinetics (using a logistic model) of 10 successive fermentations using serially repitched yeast. The 10 fermentations all behaved quite similarly, with only minor differences in fermentation rate. There also didn’t seem to be any relationship between fermentation rate and number of times the yeast was repitched. This shows that you can easily reuse your yeast for over 10 generations without any significant effects on your fermentation (as long as your hygiene practices are good). Positive linear correlations were found though between the concentrations of several aroma compounds and the maximum fermentation rate (the B parameter in the model). Faster fermentation leads to more isoamyl acetate, isobutyl acetate, ethyl hexanoate and ethyl octanoate, which of course is something that seems logical as these compounds are synthesized from metabolic intermediates.

  • Omics analysis revealed multiple stress responses of lager yeast in the process of autolysis by Jinjing Wang

Jinjing had studied the yeast responses associated with autolysis by performing proteomic and transcriptomic analysis on yeast strains with different tendencies to autolyse. She also presented various methods for the quantification of autolysis, including measuring total protein in beer, the stability of the redox potential and nucleic acid release. Using microarray analysis they had identified a range of genes that were down- and upregulated in yeast strains that showed high tendency for autolysis (e.g. RLM1 and UBC4). To confirm the roles of RLM1 and UBC4 in the autolysis process, these genes were both knocked out and overexpressed in a production strain. Overexpression of RLM1 and knocking out UBC4 led to increased autolysis. However, one must keep in mind that autolysis is a complex process that is influenced by a range of cell functions and genes.

  • Energy state model for bottling plants by Isabel Osterroth

Isabel held the only presentation in the ‘Packaging’ topic, and she talked about an energy state model which she had developed for bottling plants. Sustainability and reducing energy use, combined with the fact that bottling plant models haven’t been made before, was the driving force for creating the model. The model described the energy use of various machines in the bottling plant depending on their operational state (machines use energy even when idle). A model that was able to predict the energy use of all the separate functions in the bottling plant was successfully created, and future work will include the use of the model for optimization purposes.

  • Impact of ascorbic acid additions in mashes by Joe Williams

Joe talked about his research on supplementing ascorbic acid to the mash, and gave a virtual tour of the pilot brewery at UC Davis. The motivation for adding ascorbic acid to the mash was to increase thiol and polyphenol formation and to decrease color development in the wort. The study was very preliminary at the moment, and it will be interesting to see the final results. The pilot brewery at UC Davis was quite impressive, featuring a six-vessel 170L brewhouse and four 20L nano-breweries. I am quite jealous.

  • Optimizing hop aroma in beer dry hopped with cascade utilizing glycosidic enzymes by Kaylyn Kirkpatrick

Kaylyn talked about the use of various glycosidic enzymes in dry hopped beer in order to release glycosidically bound aroma compounds. She had tested a range of commercial Rapidase enzymes and what effect they had on the concentrations of various hop aroma compounds in a beer dry hopped with Cascade. The addition of these enzymes seems to have had quite little effect on linalool concentrations, but the concentration of geranyl acetate seems to have been enhanced with the ‘Rapidase Hoptimase’ enzyme. Their sensory panel also noticed an increase in ‘tropical fruit’-like aroma, which could be attributed to several compounds that weren’t quantified in this experiment. It seems like an interesting idea though; using e.g. Cascade in combination with a glycosidic enzyme to replicate the aroma profile of some of the modern aroma hops (e.g. Citra). Not sure how economical such a solution is though?

  • Investigating sources of variation during dry-hopping by Daniel Vollmer

Daniel talked about methods to reduce the amount of variability between replicates in dry hopping experiments. Daniel had noticed in earlier experiments that there was quite large variation between his replicates during dry hopping experiments at pilot-scale, and thus attempted to locate sources for this variation. One of the key findings was that oxygen pickup has a large (negative) effect on hop aroma intensity, and this seemed to have been one of the largest sources of variation. Other sources was the raw material (i.e. the hops cones), which for future experiments will be ground. Another interesting observation, which I mentioned already in the summary of Tom Shellhammer’s keynote lecture, was that there is huge variability in oil content within the same hop cultivar (e.g. Cascade) from different farms. Also very interesting, as I mentioned, was that there seemed to have been no correlation between oil content and aroma intensity. So there are clearly other factors that affect hop aroma intensity as well.

Report from the 5th Young Scientists Symposium in Chico (Part 1)

I apologize again for the inactivity on the blog. I haven’t been brewing much the last half a year. The wife and I bought a house in the end of last year and we’ve been renovating it since. We finally moved in a couple of weeks ago, and have started settling in. So soon I’ll be able to return to brewing again! Anyways, last week I attended the 5th International Young Scientists Symposium on Malting, Brewing and Distilling, which was arranged at Sierra Nevada’s brewery from April 21-23, 2016 in Chico, California, USA. First of all I want to thank Ken Grossman, Sierra Nevada, Charlie Bamforth and all the other organizers for a fantastic conference (especially Sierra Nevada for their generosity)! The conference featured great scientific and social program, awesome food, a relaxed atmosphere, amazing people and delicious beer! I myself presented some of the recent research we’ve been conducting on lager yeast hybrids at VTT the past year (I’ll post a link to the presentation slides soon!). To sum up, we’ve been looking at how the ploidy of new lager yeasts affect their phenotypical properties. I’ll be writing up a more detailed post on this particular research soon, as we just had a manuscript on this work accepted.


As I mentioned, there were a lot of interesting presentations during the conference! I thought I’d write some short notes / summaries of all the presentations in case you are interested. Since there were a lot of presentations, I’m splitting this post into three parts. Anyways, here is the first third of the summaries:

  • Keynote: How Craft Brewing is Transforming the Way We Think About Hops and Hop Flavor by Tom Shellhammer

Tom opened the conference with an interesting talk on the current situation of hop use in the craft industry and hop research at OSU. Craft brewers are using more and more of the global hop production, which also has shifted from being ‘bitter hop’-dominated to being ‘aroma hop’-dominated. Tom also reminded the audience that 1 IBU is not the same as 1 ppm iso-alpha acid. This is particularly relevant with heavily dry hopped beers, where oxidized alpha acids (which are bitter, but not as bitter as iso-alpha acids) can influence the IBU value. In some commercial (dry hopped) beers that had been analysed at OSU, they had observed very high levels of oxidized alpha acids. Another point that was brought up, was that the perceived bitterness gets saturated at high IBU levels (i.e. very little sensorial difference between a 80 IBU beer and a 100 IBU beer). Tom also showed a very interesting figure (which Daniel Vollmer showed again later in his presentation), showing the relationship between hop oil content in Cascade hops sourced from different farms and the hop aroma intensity in beers brewed with these hops (determined by a sensory panel). What was extremely interesting was that there seemed to be no correlation what so ever. The beer brewed with the Cascade hops with lowest oil content actually seemed to have one of the highest aroma intensities. Furthermore, many of the Cascade hops that had the highest oil contents produced beers with the lowest aroma intensities. This just shows that blindly looking at hop oil contents in hops doesn’t actually tell very much about what kind of hop aroma it will give to the beer. If I remember correctly, Tom also suggested that there was no correlation between linalool or myrcene concentrations and the hop aroma intensity either, meaning that there are other key aroma compounds responsible for hop aroma out there that still need to be identified.

  • Towards the release of a 2-row barley variety for California craft malting and brewing by Joshua Hegarty

Joshua talked about how they have attempted to breed a 2-row barley variety that would be suitable for the ‘harsh’ growing conditions in California. These include an abundance of plant pathogens and dry conditions. They had crossed different parent strains, and selected superior varieties which they had then tested in the field. The new breeding lines had shown good yields and malting quality in the field trials. Using gene mapping they had also found several regions associated with disease tolerance in barley.

  • Impact of barley varieties on malt and beer flavor by Lindsay Barr

Lindsay presented some research on the influence of barley varieties on malt and beer flavour that had been carried out at the New Belgium Brewing. Barley variety seems to have quite a big influence on both wort and beer flavour (at least according to their sensory panel). However, there didn’t seem to be any correlation between the flavours that were observed in the wort and the beer. Beer age seemed to have had a bigger impact on the beer flavour than the barley variety.

  • Selective pressurized liquid extraction of hop oil from hop cones by Katy Orr

Katy talked about some of the hop-related research that had been done at Sierra Nevada Brewing. Her background was in environmental chemistry, where she had used different extraction methods to quantify hydrocarbons from environmental samples. Here, she talked about how they had tested two different extraction methods, selective pressurized liquid extraction and Likens Nickerson distillation, to test the efficiency of their hop torpedo. Both methods seemed to have yielded quite similar results for some of the compounds that were analysed. However, the main points that were brought up were that the extracted amount does not equal the actual contents and subsequently the importance of good internal standards (that behave chemically and physically as similarly as the compound of interest as possible).

  • Pro-oxidative effects on the storage stability of German Perle and Czech Saaz pellet hops by Mark Zunkel

Mark had compared the stability of Perle and Saaz hops exposed to oxygen at room temperature during a 9 month period. The hop storage index (HSI; which measures the loss of alpha and beta acids spectrophotometrically) of Perle remained quite stable for around 4 months, after which there was a more rapid loss of the hop acids. Saaz seemed to have remained slightly more stable than Perle, but also experienced a more rapid loss in the latter half of the experiment. Unsurprisingly, both hop varieties suffered a rapid loss of hop oil in the pro-oxidative environment (50% loss of hop oil in a week). This just shows that aroma hops should be stored cold and without the presence of oxygen!

  • The effect of hopping regime, cultivar and yeast ß-glucosidase activity on terpene alcohol levels in beer by Daniel Sharp

Daniel talked about the research he had been doing on the release of hop terpenes into beer from hop glycosides. This is an interesting topic for brewers interested in hop aroma, as aroma-active compounds can potentially be released during fermentation through the hydrolysis of hop-derived glycosides in the beer. He had tested the beta-glucosidase activity of a wide range of brewing yeast strains, and then selected strains with high and low activity. Surprisingly, beta-glucosidase activity didn’t seemed to affect the maximum hydrolysis level that was achieved during fermentation (and this level was much lower than the positive control where purified enzyme was added to wort). It just took a slightly longer time to reach this level with the low activity strain. Daniel didn’t seem to see any correlation between beta-glucosidase activity and the amount of aglycones in the beer. Higher glycoside extraction was achieved with whirlpool and dry hopping compared to kettle hopping. Some varieties that seemed to be high in glycosides were Columbus, Centennial, Simcoe and Summit.

  • Creating a gin utilizing novel Scottish Botanicals: A University-Industry collaboration by Margaux Huismann

Margaux talked about her MSc project, which was carried out as a collaboration between Edinburgh Gin and ICBD. During the project, she and 3 other students had developed a gin featuring Scottish (coastal) botanicals. They went to the Scottish coast to forage for interesting botanicals, and then distilled them in lab scale to develop a recipe. The recipe was then used at larger scale at the distillery to produce a commercial product. One botanical in particular, Bladderwrack, seemed to have given off a strong ‘fishy’ aroma during distillation, and its volatile aroma compounds were analysed in more detail. We later got to try the actual gin, and it was really nice (not at all as salty or ‘fishy’ as I first was expecting). Thanks Margaux!

  • Keynote: Impact of brewing practice on yeast performance by Katherine Smart

Katherine talked about some of the research she has been doing the last 15 years. This research has been focused mainly on repitching, yeast viability, stress tolerance and petite mutants. Most interesting to me was the work on why ‘1st Generation’ yeast (i.e. yeast that have already undergone one fermentation) seem to start fermentation faster than ‘0 Generation’ yeast (i.e. yeast that come straight out of the propagator). One cause, is that G1 yeast bud faster than G0 yeast (i.e. enter the replication cycle faster) and (if I remember correctly) are able to use glucose faster from the wort. G1 yeast also seem to use less FAN from the wort, which I found interesting (less nitrogen demand or more biosynthesis?). They had also used high-throughput screening systems to isolate osmo- and ethanol tolerant strains. A quite interesting remark was that strains are seldom good at both, i.e. an osmotolerant strain is rarely ethanol tolerant as well. One good point that was made regarding these high-throughput systems is that you find what you are looking for. These isolates may have high tolerance, but may otherwise perform badly in wort or produce off-flavours.

  • Cambridge Prize Lecture: The Influence of Yeast Handling on Petite Mutant Formation by Stephen Lawrence

Congratulations to Stephen for winning the Cambridge Prize! Stephen talked about the research he had carried out, which won him the Cambridge Prize. His research was focused on petite (or respiratory-deficient) mutants in brewing, and during his presentation he also talked about various stresses the yeast are subjected to during fermentation. Petite mutants (i.e. cells with damaged mitochondrial DNA) form during fermentation as a result of fermentation stresses, and these can accumulate when yeast is repitched for several generations. These petites perform worse in several regards compared to wild type cells, so their accumulation is not desirable from a brewer’s point of view. Some interesting points that were brought up, were that older cells (i.e. cells with more budding scars) were more susceptible to petite formation and that lower mtDNA copy numbers actually didn’t increase the likelihood of petite formation (e.g. older cells tend to have more mtDNA copy numbers). This seems to suggest that the accumulation of mtDNA damage has a higher impact on petite formation than the copy numbers of mtDNA. This was a very interesting talk, and the topic still seems to be quite poorly understood. It will be interesting to follow the topic in the future.


Parts two and three will be posted during next week!

Homebrew: Fresh Hop Ale

I collected this year’s hop harvest two weeks ago, and today we finally brewed a beer with them. This is the first time I’m using homegrown hops, so am really looking forward to tasting the final beer! Technically, this wasn’t actually a fresh hop ale, since I dried the hop cones before using them – but I’ll call this a fresh hop ale anyways. Since I’m not sure about the alpha acid content of these hops, we decided to use them only as flame-out hops, and instead use some Herkules at the beginning of the boil. This way we will hopefully extract the maximum aroma out of them as well. The homegrown hops weren’t very aromatic, so I’m expecting mostly grassy flavors and less of the typical citrus and pine resin. But hopefully I’ll be positively surprised! The post-boil wort had a slight perfume-like tone, which might have been hop-derived. For the malt bill, we went with a very simple 90% Maris Otter and 10% Carapils to an OG of around 1.050. This should let the hops shine (if they do). For the yeast, we used a really fruity ale strain that I’ve developed at work (more about that in a future post). I’m hoping it will compliment the hops, and make for a refreshing and crisp fresh hop ale! We will see in a couple of weeks!


This was the first time we brewed at our ‘new’ basement brewery (we moved there 1.5 years ago – yes, we’ve been extremely slow with the renovation), and I’m happy to say that everything went really smoothly. We were done in 5 hours and 30 minutes, reached almost 70% brewhouse efficiency, and the ventilation system (a really powerful inline exhaust fan) worked amazingly well. Looking forward to brewing a bit more regularly from now on!

[beerxml recipe=http://beer.suregork.com/wp-content/uploads/2015/10/freshhop.xml metric=true cache=-1]

Hop Harvest

Today I harvested the cones off the hop plants in my yard. They might not have been perfectly ripe, but we have some cold nights coming up and I didn’t want to risk them getting destroyed by frost. They were smelling really good though! I collected 400 grams of hops in total, which after drying will probably fall to around 80 grams. To dry the cones, I spread them out in three layers on a steel mesh and put a fan underneath to recirculate the air. I’m hoping to brew a beer with them in a couple of weeks!



Hops Growing Update

A couple of weeks ago, my first-year hop plants finally started producing cones, and today they were looking like this:



We were hit by some really cold weather (temperature dropped to 0C the previous night), so the cones might not develop properly. If they do develop, I will be brewing a fresh hopped ale at some point in the middle/end of October.

Hops Growing Update

I planted some hop rhizomes earlier this year, and they’ve been growing extremely well overall. Shortly after I moved the plants from the indoor pots to the garden in April, we had a spell of some really cold nights, which unfortunately killed both Centennial plants. The other 4 (Cascade, Chinook, Columbus and Nugget) survived and have been growing well (the bines are at least 3 meters long and they’ve reached the roof). Not getting my hopes up for a first-year harvest, but hopefully some of the plants will at least produce something!


Growing hops at home

I (pre-)ordered some hop rhizomes earlier this spring (Cascade, Centennial, Chinook, Columbus and Nugget), and on Tuesday a package arrived at my door. I planted some Cascade and Chinook over at our summer cottage two years ago, but they haven’t really done that well. The Chinook grew extremely well the first summer and even gave quite a large first-year harvest, but it unfortunately died during the first cold winter. The Cascade has grown poorly both years, and hasn’t given many cones either year. I decided to start fresh this year, and will also plant them in our garden at home instead (so I can monitor them more easily). Like last time, I started by planting the rhizomes in pots and keeping them indoors (I know it is not optimal to place the rhizomes this close together, but they will be moved before they are allowed to tangle together). As soon as the weather gets a little warmer (probably in a month), I will move them outside and give them more space. The rhizomes all seemed to be in good condition when they arrived, so am hopeful that at least something will grow.

CO2 Hop Extract

I recently bought some bulk CO2 hop extract, as I was interested in trying it out. Many craft brewers (e.g. Russian River, The Alchemist, and Lagunitas) use hop extract for bittering additions in their heavily hopped beers, such as Imperial IPAs, to minimize the amount of hop material in the boil. This has two benefits, as when large amounts of hops are boiled for long times, there is a risk of extracting vegetative flavours, and less hop material also means less wort loss to trub and an easier transfer to the fermenting vessel. I haven’t seen CO2 hop extract available for sale anywhere in Europe, so I ordered a 100 g can from the USA for 21$ or about 15€. Shipping was 17$ or 12.5€, but I added in 500 g of 2012 harvest pellet hops as well. Not too bad, but not super cheap either. The 100 g can should last for about 20 batches though (5 g of the 61% AA hop extract yields about 50 IBU in a 19L batch of medium strength wort). I bought some cheap syringes, syringe caps, and needles from a German eBay shop, so I could split the extract into more manageable doses. I was surprised to notice that the can contained probably closer to 150 g of extract (since the density should be quite close to 1 g/ml), since I even ran out of syringes having filled nine 5 ml syringes and eight 10 ml syringes. They should store at least 5 years if kept air-tight in the refrigerator, so these will last some time. I will mostly be using them for IPAs and IIPAs. As soon as we get the new ‘brewery’ running I will probably brew up a batch up IIPA for the summer, as I also have an incoming order of 2012 harvest Amarillo, Cascade, Centennial, Citra, Columbus, Mosaic and Simcoe coming in. Should be a hoppy homebrew year!

Homegrown Hop Harvest 2012

I planted two Cascade and Chinook rhizomes in early June this year, and they have grown surprisingly well during the summer. I wasn’t expecting any harvest the first year, but to my great surprise, both produced a small amount of hop flowers and cones. I harvested the cones today and spread them out on the heated floor to dry. The wet weight of the cones was 48 grams, so not that much, but still better than nothing, which I had expected. Will probably use these to aroma hop a small batch of Pale Ale.


Hop Science III: Bitterness

Disclaimer: Please note that I wrote this in 2012, and the field has advanced considerably since then, so information might not be up-to-date!

This is part III of the mini-essay on hop science and factors influencing hop flavor and aroma in beer. The previous part (II – Hop Oil Composition) can be found here. The topic for this post will be bitterness and how the major components of hop resin contributes to the flavor and bitterness of beer.

One of the primary uses (if not the most important use) of hops in beer brewing, is to contribute bitterness to the flavor. Hops have been used for centuries as the prime bittering agent in beer, but hops have not been the sole or original bittering agent for beer, as a variety of different herbs were used far before the use of hops became popularized. Bitterness is usually thought of as an acquired taste, i.e. something you get used to and learn to like, and bitterness perception also varies from person to person, as some can be more sensitive to it compared to others. It is also usually agreed upon that humans have through evolution learned to become alert to bitter food, as it may be a sign of poison or a possible dietary danger, and hence very bitter beers can usually be very off-putting to a person that is not used to the flavor. Bitterness research is therefore complex, but thankfully a lot of knowledge has been gained in the area.

The general structure of an alpha-acid. For humulone, R=CH2CH(CH3)2, for cohumulone, R=CH(CH3)2, and for adhumulone, R=CH(CH3)CH2CH3.

The major contributors to hop bitterness in beer are isomerized alpha-acids, however it has been found that these are not the only compounds that contribute to the bitterness (others include products derived from beta acids and polyphenols). The iso-alpha-acids are derived from alpha-acids present in the lupulin glands of the hop flower cones, and these alpha-acids are then isomerized primarily during the wort boil into iso-alpha-acids. Dried hops typically contain around 2-20% alpha-acids by weight. The three major alpha-acids present in hop resin are humulone, cohumulone and adhumulone, while two minor alpha-acids, prehumulone and posthumulone, have also been identified. These alpha-acids are all weak acids (a pKa of around 5.5), structurally related (they differ slightly via their side-chain), and are poorly soluble in acidic aqueous solutions (approximately 50 ppm for humulone in 25° C water at pH 5 and 10 ppm for humulone in 25° C water at pH 4, while a limit of 14 ppm for alpha-acids in a lager beer has also been reported). Non-isomerized alpha-acids are usually regarded as having only a slight bitter flavor, which can be tested by tasting raw hop cones or pellets, but the bitter flavor of the non-isomerized alpha-acids is not nearly as intense as the flavor of their isomerized counterparts (which is nine times stronger). The flavor threshold is also close to the maximum solubility of the alpha-acids in beer (Intelmann et al. (2009) recorded flavor thresholds of 8 ppm, 6 ppm, and 8 ppm for humulone, cohumulone and adhumulone respectively in water by a trained sensory panel, while the half-maximum bitterness intensity concentration for humulone was over 180 ppm), meaning they don’t contribute to any of the bitterness in beer.

The isomerization reaction of humulone to iso-humulone.

Thermal isomerization of the alpha-acids to iso-alpha-acids occurs via an acyloin-type ring contraction, and the reaction rate is increased with increasing temperatures. Isomerization still occurs at pre-boil temperatures, but the reaction rate at 90° C is approximately half of the reaction rate at 100° C. Isomerization forms two epimers, a trans– and a cis-iso-alpha-acid (forming in a ratio of around 1:3 in wort), meaning 6 different iso-alpha-acids are formed from the 3 major alpha-acids present in hop resin. The trans-isomers are both less bitter in flavor and less stable (i.e. more prone to oxidation) than the cis-isomers, meaning the cis-isomers are usually more desired. Foam-stabilizing properties of the cis-isomer are though slightly weaker compared to the trans-isomer. The iso-alpha-acids have pKa values around 3.0 and have much higher solubility in wort than the non-isomerized alpha-acids (predicted solubility of trans-isohumulone in water 72.5 ppm). This, along with the fact that the flavor threshold of iso-alpha-acids in water is around 6 ppm, explains why the iso-alpha-acids contribute for the majority of the bitterness in beer. The total iso-alpha-acid concentration can reach over 100 ppm in more bitter beers. The limiting factors include the poor solubility of the alpha-acids, the reaction rate of the isomerization, and the reaction rate of the degradation of iso-alpha-acids. Prolonged boil times (over approximately two hours) do not increase the iso-alpha-acid content of the wort, since after this the rate of iso-alpha-acid degradation exceeds that of alpha-acid isomerization, and the iso-alpha-acid starts to decrease. Hence, the maximum amount of bitterness one can get from a 60-minute boil addition (independent of the size of the addition) is approximately 50 IBU (see this Basic Brewing Podcast and report). If one thrives for a higher bitterness, it must be achieved from a combination of hop additions.

Table 1 – Alpha-acid and Cohumulone content of various hop varieties
[table id=3 /]

The type of alpha-acid also presumably influences the quality of bitterness, as it is generally believed that iso-cohumulone contributes a harsher and more unpleasant bitterness compared to iso-humulone. However, recent studies seem to indicate that there isn’t any difference in bittering quality between the two alpha-acids. Because of a slightly higher polarity, cohumulone and iso-cohumulone have slightly higher solubility in wort compared to humulone and iso-humulone, meaning they are usually present in slightly higher quantities. Table 1 contains a table with the typical alpha-acid percentages and cohumulone contents of various hop varieties. Looking at the table, it becomes evident that hop varieties such as Simcoe, Horizon and Warrior are preferred, if one is after a hop variety that has a low-cohumulone, yet high alpha-acid, content.

A quartz cuvette used for IBU assays. (Source)

Beer bitterness is usually measured in International Bitterness Units (IBU), or the almost identical European Bitterness Units (EBU), and it is a measure of the absorbance of a solvent extracted sample of beer at 275 nm. The absorbance is of course dependent on the amount of isomerized alpha-acids present in the sample. 1 IBU approximately converts into to 0.7 ppm iso-alpha-acid, though this depends on the presence of other bittering compouds. The IBU assay is carried out by first acidifying a sample of beer with HCl (to make the bittering acids more hydrophobic), then adding twice the volume of 2,2,4-trimethylpentane (also known as iso-octane). The mixture is then shaken thoroughly for 15 minutes, to allow as much of the (now hydrophobic) acids to transfer to the organic phase. The absorbance of the trimethylpentane phase (organic) is then measured at 275 nm in an ultraviolet spectrophotometer (against a blank). The measured absorbance is then multiplied by 50 to convert into Bitterness Units. Since the absorbance is affected by oxidation products and beta acids as well, it doesn’t give a definite picture of the amount of isomerized alpha-acids in the beer, nor the actual perceived bitterness. HPLC can instead be used to measure the amount of, and types of, isomerized alpha-acids in beer, but it requires more labor and time, as well as more costly equipment.

The general structure of an beta-acid. For lupulone, R=CH2CH(CH3)2, for colupulone, R=CH(CH3)2, and for adlupulone, R=CH(CH3)CH2CH3.

As mentioned, there are also beta-acids present in the lupulin glands of hop cones, but it is generally regarded that these don’t have a large effect on the bitterness in beer. Like the alpha-acids, there are five types typically found in hop resin (lupulone, colupulone, adlupulone, prelupulone and postlupulone) and they are structurally related weak acids (pKa of around 6). The beta-acid content of hops is usually around 1-10% by weight, and the beta-acids usually precipitate out of the wort and beer. Some of the beta-acids can though undergo transformations into bitter-tasting products such as cohulupone, hulupinic acid, nortricyclocolupone, two tricyclocolupone epimers, two dehydrotricyclocolupone epimers, two hydroxytricyclocolupone epimers, and two hydroperoxytricyclocolupone epimers. These transformation products have flavour thresholds only slightly above those of iso-alpha-acids, meaning they most likely in some way contribute to the bitterness of beer. Another series of compounds most likely contributing to the bitterness of beer are polyphenols, which are naturally found in hops, as well as malt. Polyphenols can lend a harsh and astringent flavour to the beer in large concentrations. These polyphenols can transfer to the beer through hop debris or dry hopping (polyphenols are regarded as the cause of dry hop haze), and hence even though dry hopping doesn’t contribute significantly to the amount of isomerized alpha-acids, they do contribute slightly to the bitterness of the beer through the introduction of polyphenols.

As mentioned above, cis-iso-alpha-acids are usually more stable than their trans-counterparts, meaning the ratio between them in beer usually is a good indicator over bitterness stability. To improve the bitterness stability of the beer, various techniques can be applied. The use of pre-isomerized hop products allows for a higher cis:trans ratio, post-boil whirlpool hopping has shown to increase bitterness stability, and high mash temperatures can lead to an increased amount of coagulated protein before to wort boiling and consequently less humulone-loss in the trub, which then leads to higher hop utilisation and bitterness stability. It has also been proposed that the pH of the beer affects iso-alpha-acid oxidation, though the exact effect is unclear. Isomerized alpha-acids can also undergo reduction, to form dihydroiso-alpha-acids (RIAA), tetrahydroiso-alpha-acids (TIAA) and hexahydroiso-alpha acids (HIAA). These acids are also bitter-tasting, and have improved stability against light and also increases foam stability. In water, RIAA is about 0.67 times as bitter as iso-alpha acids, TIAA 2.03 times and HIAA 1.15 times. These reduced iso-alpha-acids have similar bittering qualities to their non-reduced counterparts, and can be added to beer e.g. post-fermentation. These reduced iso-alpha-acids are usually produced using sodium borohydride, and RIAA and TIAA are available on the market for brewers.

As can be seen, hop bitterness is also a complex subject, and the traditional views on hop boiling and bitterness are slightly flawed (e.g. very high IBU values can be achieved by adding hops only post-boil). The next part will also be the final part of the mini-essay, and it will be focusing on combining the theory from this and the previous part, and applying it to actual beer brewing. I should hopefully release it in about two weeks, together with information from a trial brew.


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